You've tried everything for SIBO and constipation. Your genes may hold the answer.

Your gut is a muscular tube that propels food from stomach to colon through coordinated contractions. Serotonin is the neurotransmitter that triggers these contractions. After serotonin is released, it must be recycled back into nerve terminals so it can fire again. The SLC6A4 gene encodes the serotonin transporter, the protein responsible for this recycling process.

The 5-HTTLPR short allele variant (carried by roughly 40% of the population) impairs serotonin reuptake. That means serotonin hangs around longer in the synapse, which sounds helpful until you realize it also desensitizes the receptors. Your gut muscles receive fewer effective signals to contract. The result is slower intestinal transit, bacterial overgrowth in the small intestine, and the sensation of perpetual constipation.

You feel blocked. You strain. Nothing moves. You increase fiber thinking it will help, but your intestines simply aren’t muscular enough to push it through. SIBO develops because bacteria have more time to ferment undigested food. The longer food sits, the more gas is produced, the more bloated you feel.

COMT metabolizes catecholamines (dopamine and norepinephrine), neurotransmitters that regulate gut motility, visceral sensation, and stress response. A slow COMT variant means catecholamines accumulate. A fast COMT variant means they’re cleared too quickly. Both create problems.

Carriers of the Met158 allele (slow COMT) make up roughly 25% of the population. They produce excess dopamine and norepinephrine, which can amplify visceral pain signaling and stress reactivity. Their gut becomes hypersensitive, anxiety worsens, and gut motility becomes erratic, alternating between slow transit and urgency. Fast COMT carriers (Val158 homozygotes, about 25%) burn through catecholamines too quickly, losing the dopamine signal needed for normal gut muscle coordination.

If you have slow COMT, eating caffeine or high-stimulant foods makes your gut worse. Your abdomen feels like it’s in constant pain. Stress paralyzes your motility. If you have fast COMT, your gut feels sluggish because the motivational neurotransmitter is depleted.

MTHFR converts dietary folate into methylfolate, the activated form your cells use to produce neurotransmitters, including serotonin and dopamine. It’s also essential for the methylation cycle, which produces SAM-e, the methyl donor that regulates gene expression throughout your body, including genes controlling inflammation and immune tolerance.

The C677T variant, carried by roughly 35% of the population (higher in certain ethnic groups), reduces enzyme efficiency by 40-70%. The A1298C variant has a smaller effect, roughly 20-30% reduction. People with these variants cannot effectively convert dietary B vitamins into the active forms their gut nerves need to signal properly. They’re eating folate, but their cells can’t use it.

You experience the symptoms of B-vitamin deficiency despite eating leafy greens: fatigue, brain fog, poor gut motility, and impaired serotonin production. Your intestines receive weaker signals to contract. Bacterial overgrowth follows. Anxiety and depression may worsen because you lack the methylated B vitamins needed for neurotransmitter synthesis.

Vitamin D doesn’t work without the VDR (vitamin D receptor) gene. Vitamin D binds to VDR, and together they regulate immune tolerance in the gut. This is critical because your small intestine contains trillions of bacteria that must be tolerated, not attacked. VDR activation tells immune cells, “These bacteria are normal. Don’t mount an inflammatory response.”

Common VDR variants, carried by roughly 50% of the population, reduce how effectively vitamin D can activate the receptor. Your immune system loses the “tolerance” signal and begins treating normal bacteria as threats, triggering inflammatory responses that damage the intestinal lining and create an environment where pathogenic bacteria thrive. SIBO develops because your compromised immunity can’t control bacterial growth.

You develop food sensitivities that weren’t there before. Your intestines become inflamed and leaky. Bacterial metabolites trigger systemic inflammation. You feel worse despite eating cleanly because your gut immune system has become hostile to the normal bacteria that should be protecting you.

NOD2 is an innate immune receptor that recognizes peptidoglycan, a component of bacterial cell walls. When NOD2 detects bacteria, it signals the immune system to mount an appropriate response: create barrier-strengthening compounds, produce antimicrobial peptides, and maintain healthy inflammation. It’s a “friend or foe” detector.

The three major NOD2 variants (R702W, G908R, 1007fs), carried by roughly 7-10% of European ancestry, severely impair this recognition function. Your immune system loses the ability to sense bacteria in the small intestine, so it can’t mount the controlled immune response needed to keep them in check. Bacteria proliferate unchecked, triggering SIBO.

You may have no obvious inflammatory markers on standard blood tests, yet your small intestine is colonized. You develop symptoms seemingly out of nowhere: bloating, gas, alternating constipation and diarrhea, food sensitivities. The bacteria aren’t inherently pathogenic. Your immune system simply can’t recognize them to regulate their population.

TRPV1 is a pain receptor found throughout your gut lining. It detects heat, acid, and irritating compounds. Normally, it fires only when there’s actual damage or genuine irritation. But TRPV1 variants create a hair-trigger response: your gut becomes hypersensitive to normal sensations that shouldn’t hurt.

Variants in TRPV1 (carried by roughly 25-30% of the population) increase sensitivity to temperature and chemical irritants in the gut. Normal food transit, normal bacterial fermentation, even normal intestinal contractions feel painful because your pain receptors fire at lower thresholds. This heightened pain signaling is misinterpreted as a need to slow motility, so the gut compensates by moving slower. Slower transit means more bacterial overgrowth, more fermentation, more triggers for the hypersensitive nerves.

You feel constant discomfort in your abdomen. You avoid foods that should be safe because they trigger pain. You’re afraid of eating because every meal triggers distension and pain. The pain creates a vicious cycle: anxiety impairs motility, slow motility causes fermentation and gas, gas triggers the hypersensitive nerves, and pain worsens anxiety.